Carboxylates, especially low molecular weight carboxylates, occur in the chemical industry during various reactions as by-products or main products. For example, methyl acetate is a typical by-product in the production of purified polyvinyl alcohol. Methyl acetate mixtures from polyvinyl alcohol plants, besides a small amount of low-boiling substances such as acetaldehyde, contain an azeotropic mixture of methyl acetate and methanol.
For chemical companies, where methyl acetate as a by-product occurs in relatively small quantities, methyl acetate is not an economically interesting product, since it cannot be profitably sold on the market. A better price can be obtained if methyl acetate is hydrolysed into acetic acid and methanol. The hydrolysis can be carried out either as a batch process or continuously, by using either a reactor in conjunction with conventional distillation or a single reactive distillation column.
The use of a reactor in conjunction with a distillation column for the hydrolysis of methyl acetate is described, for example, in U.S. Pat. No. 4,352,940. This type of hydrolysis of methyl acetate has several disadvantages: (a) since the reaction is an equilibrium reaction with a small equilibrium constant, the yield of reaction product is small, (b) because of the azeotropic mixture which is set up in the reaction mixture between methyl acetate/water, on the one hand, and methyl acetate/methanol, on the other, several distillation stages are needed after the reaction. This leads to high investment and running costs, (c) corrosion problems occur because homogeneous catalysts such as sulphuric and hydrochloric acid are used in the known conventional processes.
U.S. Pat. No. 5,113,015 discloses a process for obtaining acetic acid from methyl acetate, in which methyl acetate and water are brought into contact in the presence of catalyst packing in a distillation column. In this case, the methyl acetate is hydrolysed into acetic acid and methanol. The resulting reaction mixture is in this case partially separated at the same time in the separation column.
U.S. Pat. No. 5,770,770 likewise discloses a process for the hydrolysis of a methyl acetate mixture in a reactive distillation column. The hydrolysis of a methyl acetate flow, which contains more than 50% methyl acetate, takes place in a reaction zone in which ion exchange packing is present. The methyl acetate mixture is supplied from below to the ion exchange packing, and water from above onto the ion exchange packing. The unreacted methyl acetate and water vapour are collected and condensed in the upper part of the reaction zone, and are then recycled back to the reaction zone. At the same time, the base is collected and separated into the hydrolysis products and impurities. The impurities are then returned to the reaction zone.
Both aforementioned U.S. patents, U.S. Pat. Nos. 5,113,015 and 5,770,770, teach the use of strongly acid ion exchangers as catalysts. In U.S. Pat. No. 5,770,770, it is proposed to put the ion exchange material in the form of Raschig rings, by adding a plastic as binder to the ion exchanger and pressing the mixture into appropriate shapes. In U.S. Pat. No. 5,113,015, the catalyst material is conserved in glass wool, and the mat formed therefrom is rolled up with a wire mesh between the layers, in order to promote circulation of the fluids. The catalyst packing material may be a compliant open-mesh substance, e.g. a metal cloth. Another usable material is a more rigid cellular monolith, which can be produced from steel, polymers or ceramic material. The catalyst packing material may, however, also be produced from corrugated metal sheets or corrugated plastic or ceramic plates.
The processes described above, which use only a single reactive distillation column, suffer from the fact that methyl acetate is only partially converted to methanol and acetic acid. The amount of methyl acetate in the product flow makes the use of at least one additional purification stage necessary, which leads to additional investment and running costs. A further problem is that the methyl acetate feed flow often contains metal ions, which poison the catalyst in the reactive distillation column. The replacement of the catalyst material in the distillation column by fresh material, however, is laborious and expensive. Furthermore, it is desirable for the hydrolysis process to be controlled independently of the methyl acetate concentration in the feed flow—in such a way that, as required by the operator e.g. of a polyvinyl alcohol plant, the acetic acid obtained has a specific water content or is virtually anhydrous.
The object of the present invention is therefore to provide an improved process for the hydrolysis of a carboxylate, especially methyl acetate, methyl formate and allyl acetate, by using a reactive distillation column. In particular, it is desirable to improve the conversion ratio of carboxylates into alcohol and carboxylic acid. A further object is to prevent poisoning of the catalyst used in the distillation column. Another object is to optimise the energy budget of the process. It is also intended to offer a process and a hydrolysis device, which allow great flexibility in terms of composition and quantity of the feed. The composition of the product flows is also intended to be controllable in a large range by the process. A further object is to improve the conversion ratio of methyl acetate into acetic acid and methanol.